selftests: drv-net: ncdevmem: make configure_channels() support combined channels

selftests: drv-net: ncdevmem: make configure_channels() support combined channels

ncdevmem tests that the kernel correctly rejects attempts
to deactivate queues with MPs bound.

Make the configure_channels() test support combined channels.
Currently it tries to set the queue counts to rx N tx N-1,
which only makes sense for devices which have IRQs per ring
type. Most modern devices used combined IRQs/channels with
both Rx and Tx queues. Since the math is total Rx == combined+Rx
setting Rx when combined is non-zero will be increasing the total
queue count, not decreasing as the test intends.

Note that the test would previously also try to set the Tx
ring count to Rx - 1, for some reason. Which would be 0
if the device has only 2 queues configured.

With this change (device with 2 queues):
  setting channel count rx:1 tx:1
  YNL set channels: Kernel error: 'requested channel counts are too low for existing memory provider setting (2)'

Reviewed-by: Mina Almasry <almasrymina@google.com>
Link: https://patch.msgid.link/20250815231513.381652-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>